The modifying effect of prediabetic status on the association of social isolation and loneliness with the risk of type 2 diabetes mellitus (T2DM) remains uncertain. We aimed to explore whether prediabetic status modifies the association of social isolation and loneliness with incident T2DM.

358,951 participants with random blood glucose < 11.1 mmol/l, hemoglobin A1c < 6.5 % and without diagnosis of diabetes from the UK Biobank were included. Prediabetes was defined by hemoglobin A1c level at 5.7–6.4 %. Social isolation and loneliness were assessed using self-reported questionnaires. The study outcome was incident T2DM.

During a median follow-up of 12.5 years, 13,213 (3.7 %) incident T2DM cases were documented. Social isolation and loneliness in subjects with normoglycemia (adjusted HR [95 %CI]: social isolation: 1.14 [1.07;1.23]; loneliness: 1.33 [1.20;1.47]) were more strongly associated with increased risk of T2DM than in those with prediabetes (adjusted HR [95 %CI]: social isolation: 0.97 [0.91;1.03]; loneliness: 1.04 [0.95;1.13]) (Both P for interaction < 0.001). Among individuals with prediabetes, alcoholic consumption (30.9 %), household income (23.3 %), healthy sleep (17.1 %), loneliness (14.9 %), and physical activity (12.6 %) mediated most of the variance in the association between social isolation and incident T2DM, while body mass index (17.9 %) and healthy sleep (17.6 %) mediated most of the variance in the association between loneliness and incident T2DM.

Social isolation and loneliness were independently associated with a higher risk of T2DM among individuals without prediabetes. Among those with prediabetes, the association of social isolation and loneliness with incident T2DM were mainly mediated by some socioeconomic and lifestyle factors.